The invention concerns an improved process for deasphalting or deresining heavy hydrocarbon fractions containing asphaltenes and/or resins, particularly fractions recovered from petroleum, shales or bituminous sands, or obtained from coal conversion.
The heavy oil fractions, or those obtained from coal, boiling above 350.degree. C. (atmospheric residues) or above 550.degree. C. (vacuum residues) contain a number of undesirable components which make their refining difficult. These components are mainly asphaltenes, resins and oil-soluble organometallic compounds containing nickel and vanadium. These organic compounds are destroyed in the hydrorefining operations conducted with hydrogen in the presence of catalysts belonging chiefly to groups Vb, VIb and VIII of the periodical classification. The nickel and vanadium compounds foul the catalysts and contribute to their quick deactivation.
For analysis purposes, the following definitions are commonly accepted:
Asphaltene means the hydrocarbon fraction which precipitates when n-heptane is added, under the standard conditions of NFT 60.115.
Resin means the fraction which precipitates when propane is added after previous removal of the asphaltenes.
However, in practice, when purifying heavy hydrocarbon fractions, solvents are used which precipitate not only the asphaltenes but also the resins, in part or in totality. This is the case, for example, of hydrocarbons having 3 to 6 carbon atoms.
In the following description, deasphalting means any operation wherein asphaltene or an asphaltene+resin mixture carrying along therewith a variable proportion of oil, is separated, the separated fraction being designated as "asphalt" for sake of brevity, as is usual in the industry.
It is well-known today that the asphaltenes contain the major part of the nickel and vanadium compounds present in the heavy oils. In a typical case, 80% of the (Ni+V) compounds contained in the heavy oil fraction are present in the "asphaltene" fraction. The remainder of the organometallic compounds is associated with the fraction identified as "resin".
A well known method to catalytically refine and hydropurify (hydrodesulfurize, hydrogenate, hydrodeoxygenate, hydrodenitrogenate) heavy fractions consists, in a first step, called a solvent deasphalting step, of eliminating the asphaltenes and a fraction of the resins (thus also the metals) by precipitating them by addition of an appropriate solvent, and then, in a second step, of hydrotreating the fraction freed from the asphaltenes and from a large portion of the resins and metals. The deasphalting operation, called solvent deasphalting, yields an asphaltene-free refined fraction containing a portion of the resins and having a low content of metals, which fraction will be called D.A.O. (deasphalted oil), and a concentrated asphaltene fraction also containing resins and oils, which will be called asphalt. The asphalt fraction, wherein all the products detrimental to refining have concentrated, cannot be utilized easily since it has not only high nickel and vanadium contents but also very high sulfur and nitrogen contents.
The economics of the so-called deasphalting process necessitate that the asphalt fraction represent a low portion of the total weight of the heavy oil fraction. To obtain this result, a relatively heavy paraffin is used as precipitation solvent. It is well known that the yield of precipitate (asphalt)decreases, everything else being unchanged, when the molecular weight of the precipitation solvent increases. The known processes preferably make use of pentane or a light gasoline as precipitation solvent.
The second characteristic feature of such a process is the ratio of the proportions of precipitation solvent and heavy fraction to purify. Since the solvent is recovered by distillation and recycled, the lower the solvent to oil ratio, the better the economics of the process.
The last important element for the expected result consists of contacting the solvent with the heavy fraction as intimately as possible. Various devices are conventionally used therefor: counter-current extraction columns provided with fixed internal walls or discs improving the contact, or better with internal moving walls or discs, as for example, the device called R D C (rotating disc contactor). In other known installations the solvent and the oil are contacted co-currently, a settler or optionally a battery of mixers-decanters being used thereafter.
Irrespective of which of the above technical solutions is selected, it is observed that the percentage of asphalt separated is far higher than the asphaltene percentage in the treated charge. When analyzing the asphalt, it is found that, in all cases corresponding to conventional techniques, the asphalt contains an oil fraction amounting to 15 to 35% by weight of the separated asphalt.
Deasphalting comprises a double operation: the dissolution of the oil into the "precipitation solvent" and the precipitation of the asphaltenes and a fraction of the resins. The presence of substantial amounts of oil in the asphalt is indicative of a poor dissolution of the oil into the solvent. A portion of the latter is carried away when precipitating the alphaltenes and the resins.